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Does Cd number change at different speeds during wind tunnel testing?
When meassure Cd or Cl of airplane/car in wind tunnel, does number change at different speeds, for example at 100km/h compare to 250km/h?
If yes,how much?
I know from theory that coefficents change with Re, and Re is change with speed, but I know what real pracital data show when you change airflow speed in wind tunnel?
Because if difference in results is big then we have to agree at what speed test the object..

Aerodynamic coefficients like $C_d$ and $C_l$ depend at least on Reynolds number, as visible for example in these plots from this answer:

So yes, they depends, among other factors, on speed. Anyway, in order to compare results from different tests, the Reynolds number has to be the same, not just the speed.
And if you are testing some specific phenomenon then also other "numbers" have to be the same: for example if compressibility effect are important, then the Mach number has to be the same. If unsteady characteristics are important (like in a flutter test), then the so called "reduced frequency" (which is also a dimensionless number) has to be the same. If the inertia of the fluid is not negligible (like in hydrodynamics), then the Froude number has to be the same. And so on.

As long as you stay subsonic these values are (near) constants.
But the forces acting on the objects do have the speed squared... That's where speed is coming into our equation.
Things do change when nearing the "sound barrier".
See the graph on this question:
Acceleration of a supersonic aircraft after breaking the sound barrier

Deviations of that sort can be caused by imperfections in the uniformity of the airflow in the tunnel throat and by the model under test being too big for the throat dimensions, leading to interference between the walls and the model. This sort of thing has to be carefully checked for anytime you get an unexpected result out of a wind tunnel test.


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What are the proper radio call-outs on uncontrolled airfields?
I know that there are many non-commercial runways that do not have an active Air Traffic Controller on-duty, and that many small aircraft use these runways for takeoff and landing, but I never actually learned what the proper communications were for this. I also do not know the terminology for it either (what it is called flying without an ATC in local airspace).
Generally, private-owned or community-owned airstrips are typically only large enough to support small aircraft and do not have an ATC. I know that pilots usually communicate with the local airspace to signal what they are doing, etc. But, again, I have no clue what the official terminology is, nor do I know the proper call-outs regarding actions, inquiries, information, etc. Does anyone mind informing me on the subject? It's better to learn something from someone rather than searching up the answer.

This seems to be a dupe of this question although it has no answers right now. You might also want to mention if you're interested in a specific country: US practices in particular are often slightly different from the rest of the world.

I believe the term you're looking for is "untowered airport", and there are quite a number of questions here about that.

Uncontrolled airfield, would be the non-USA term.

I can only speak as to the situation in the US, I don't know what the rules are elsewhere.
When operating at an untowered airfield in the US you're not actually required to have a radio at all. Thus all those restored vintage biplanes and other radioless aircraft can operate perfectly legally from an untowered field without having to get any kind of special permission.
If you do have a radio, you're strongly encouraged to monitor and periodically report on the CTAF (Common Traffic Advisory Frequency) assigned to that airport when you're within 10 miles of it. Such communications are detailed in AIM 4-1-9, but the standard format is basically "<airport name>*, <your id>** <your position>, <your intentions>, <airport name>*". For instance, "City airport, Skyhawk 1234A (or 'white and blue Skyhawk') is 5 miles to the North, going to overfly the airport at 2,000 feet and enter the left downwind for runway 27, City airport."
* Since the range of VHF communications varies, it's entirely possible that two airports with the same CTAF might be in range of each other, so saying the airport name twice, once at the beginning and once at the end, helps reduce confusion between nearby airports.
** There's an ongoing debate in the aviation community about whether to use your tail number to identify yourself, since that's guaranteed unique, or a description of your plane, since that's easier to tell from a distance.

I would definitely use description of plane, since it is far more useful information to the receiver of the broadcast than tail number. So make-model-colour would be my choice.

It's true that some pilots like to debate using the tail number or not, however you might like to mention that the FAA's position is that you must use it in calls. They're fine with pilots adding color to that, but the tail number is always required. The details are in AC 90-66.

The specifics vary a bit around the world, but the basic call anywhere is a position call; this is a radio call on a common frequency or "in the blind" to alert listening aircraft to one's position.  It will include the party addressed, identification, location, generally from a known reference point, one's altitude, and direction of flight or intentions.  If approaching a uncontrolled field to land, for example, one might say "Coolaroy traffic (party addressed), Cessna 12345 (identification) is ten miles south (location) at five thousand feet (altitude), northbound (direction), landing Boogan (intentions)."
Operations to and from runways will use standard terminology  about movement on the surface or in the air, using the same basic format as a position report.  This may include position in the traffic pattern (downwind, base leg, final approach, etc, or taxiing or taking off.
Calls may include calls to other traffic to to coordinate.  Traffic may announce a longer downwind leg to accommodate another airplane, or coordinate the entry into a traffic pattern. Aircraft may also call out identificatin of another aircraft, to put everyone on the "same page," or in other words, to enhance understanding and clarity.
When in doubt, it's best to be as clear as possible.  I can't tell what the registration number is on another aircraft at any distance I'm likely to see in flight, but I may be able to tell the color.  If the other airplane says "Bisquick Traffic, red Cessna 469Z is left downwind for runway 18, it helps me identify which aircraft on the downwind is talking; there may be several.  If see the red one, I have a better idea.  Conversely, I might ask, "are you the red one?"  A little plain communication goes a long way.  The general rule is to minimize conversation and excess language: don't use any more radio time than necessary, because it may be the words you miss that are the most important ones spoken.  Someone might have called a position or intention or even an emergency during the time someone was talking too much.   The general conventions and concise use of the radio are fairly universal.

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Does the Airfoil Tools Cd/Alpha Curve include both induced and parasite drag?
I'm looking at Cd / Alpha for the SD7062 Profile:
http://airfoiltools.com/airfoil/details?airfoil=sd7062-il
Wondering if it includes both induced and parasite drag. How does one tell?

To answer the question you have to look at the source:

Induced drag is a drag type which appears on wings with finite span. The polars given are for a 2D profile section and therefore without induced drag.
What about the parasitic drag? Using Wikipedia's definition only skin friction and form drag are taken into account. Interference drag, which is a 3D drag type dependent on the position of other bodies, can't be included either.

That makes absolute sense! Thank you! My next questions was how to calculate drag taking aspect ratio into account, which the typical drag formula doesn't take into account.

In https://www.grc.nasa.gov/WWW/K-12/airplane/induced.html there is a formula for the induced drag. If it doesn't answer your question post a new one explaining what do you want to know.

Thank you for setting me on the right track @mns. My more detailed question is now here: http://aviation.stackexchange.com/questions/35176/how-do-you-calculate-total-drag-on-a-wing

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Aircraft lifetime and maintenance
Since it cannot be expected that the materials used for aircraft contruction have infinite lifetime and sooner or later they will deteriorate, how does aircraft maintenance work?
Are there schedules based on which certain components have to be checked or substituted in order to prolong the aircraft lifetime? Will the aircraft be unusable anyway after a certain amount of flight hours?

If I understand your question correctly, see here, here, and here.

heavily edited in order to improve readibility (I should have maintained the OP intentions)

I've closed this as a duplicate of the most-similar question (Is there any limit on the age of an aircraft?) - If you don't think that one answers your question leave a comment and I can reopen this question

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What connector standard is used for aviation headsets?
The typical aviation headset features two cords with connectors that at first glance are identical. One is a standard 1/4" (0.25 inch) stereo audio jack.
What kind of connector is the slightly smaller, slightly narrower one?

Does this answer your question?

Totally OT, I know, but I just had a vision of a pilot bopping down the street with his DCs plugged into his phone (via a ¼" to 3.5mm adapter), jammin' to some tunes. Would put those kids with their wimpy Beats™ to shame!

It's for the microphone (the standard sized one is for the head phones). They are different size so the pilot knows to try the other hole if it doesn't fit.
From wikipedia:

Commercial and general aviation (GA) civil airplane headset plugs are
  similar, but not identical. A standard 1⁄4 in monaural plug, type
  PJ-055, is used for headphones, paired with special tip-ring-sleeve,
  0.206 inch diameter plug, type PJ-068, for the microphone. On the microphone plug the Ring is used for the microphone 'hot' and the
  sleeve is common or microphone 'Lo'.


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What type of altimeter is this?
What type Altimeter is an SA-91? It has three pointers 100, 1000, 10000.
Don't know if it is Kollsman. I can't find any info as far as repairing unit.

Are you asking who the manufacturer is, or its principle of operation? I'm guessing you want to know who makes it? What aircraft is this from?

are there any markings on the back? A picture of the back might be helpful.

It does appear to be a sensitive altimeter with a kollsman type adjustment, though the kollsman window is blacked out—perhaps the adjustment knob has been turned down too far. In other ways it does appear to be a Kollsman brand altimeter, markings, shape, and milling. Kollsman brand often had the name stamped into the metal at the back of the instrument as well as a tag, also on the back. Additional photos would be very helpful.

It seems to be a C-12 or C-13 sensitive altimeter (SA) from Kollsman Instrument Division / Square D Company, manufactured circa 1940:

(Source)
The only difference seems to be the marking "ALTITUDE" replaced by "SA-91". As this is the 39th in the series, this could be the initial marking, later replaced by the more standard one. I don't see what could be "91" for.
The Kollsman window is black (as commented) because the setting dial has been turned past maximum pressure reference, as indicated by the bug between 7 and 8. The window value and this bug are linked.
The brown color seem to be related to phosphorescent markings and needles.
The C-12 has apparently many variants, likely designed on demand, especially for the USAF, like this one with alternate colors for the marks:

C-12 manufactured by C.G. Conn Ltd., (source)
C.G. Conn Ltd. is a music instruments company (saxophones), but at war time they provided aircraft instruments to the USAF (flying on instruments finally explained!).
Kollsman published handbooks about their instruments. Here is one from 1938. It doesn't cover this model of altimeter, but can be helpful though:

(Source)

Someone just gave me a Kollsman C-12 with the alternating white/blue number markings that looks almost identical to the C. G. Conn you linked to, but mine is manufactured by Square D Company and "KOLLSMAN" is embossed on the back. Per the deatails on a spec place screwed to the back, it's a Type C-12, MFRS Part NO 671 BK-010, CONTR NO AF-33186 and SER NO (serial) 43[unreadable]. My co-worker gave it to me, and supposedly his dad's friend, who was a WWII pilot (deceased) gave it to him. I'd love to know what AC it came out of. Do you know which AC used the White/Blue variant like that?

FWIW I thought I could look up the coworker's dad's friend and assume it came out of an AC he flew in the war, but unfortunately his last name, "Young" is a terrible search term. Also the first name "Larry" isn't getting me anywhere, either, but it may be short for Lawrence. (It also may be a middle name..)

@elrobis: 43 is the year of manufacturing. From here: "The C-12 Altimeter was used across all aircraft of the Air Corps US Army, including heavy bombers B-17 and B-24, medium bombers, B-25 and B-26, Fighters P-38, P-39, P-40, P-47, and P-51, and transports C-46 and C-47, according to the Maintenance Interchangeability Chart dated Dec 1943."

The altimeter was used in a Nuclear Weapon test set neither a CAT or FAT test set. It was used to test the BARO switches in the weapon. The "SA" printed on the dial was for Sandia Apparatus

To me this is very plausible, Sandia (SNL) was in charge of the baro fuse to replace the radar fuse and the phrase Sandia apparatus has been used for commercially available instruments with additional Sandia-provided requirements. Can you provide some reference?

Of interest is the serial number on the face of the instrument.  Yes it appears to be the Kollsman type but lacks the barometric readout in the "Kollsman window".
I suspect that you have an early prototype of the Kollsman.  At least that is my best guess.  I'm not an expert on antique altimeters but just a knowledgeable pilot and aviation enthusiast.

authors: Андрей Меняйлов

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Is it sensible to harness wind energy on an airplane?
Is it sensible to harness wind energy on an airplane?
Actually the idea is to build an electric aircraft whose power comes from solar and from wind while the plane is landing. instead of using spoilers as airbrakes, these wind turbines separately mounted could add drag and harness energy at the same time . In other words, regenerative braking.

Normally all aircraft harness wind energy when flying with the wind, and not against it. In such situation, the energy of the wind contributes to kinetic energy of an aircraft directly and with astounding near-100% effectiveness. But usually such energy input alone is insufficient to keep flying (except kites; e.g. gliders need 'meteorological' vertical lift).

It helps to realize that the wind turbine provides drag that the engines have to compensate. So you are delivering mechanical energy from the engines to the turbine, with air as the intermediate medium. Ais is not a good medium for that. It's a lot more efficient to not have any intermediate at all, and stick the generator straight on the engine.

@kubanczyk:  Planes harness wind power in both directions.  When flying with the wind, they get increased ground-speed for the same power.  When flying against the wind, they get the same lift for less power, but with less ground speed.

@abelenky There are physical definitions of what words like energy and power mean. In level flight, lift needs zero energy input (as does any force contributing to zero position change).

One situation where it makes sense is a helicopter with engine failure. In autorotation, energy can be harnessed during descent and stored in the blades' momentum. It comes in useful at sufficiently low altitude.

You might want to look at Makani Power.

While this would work for regenerative braking: aircraft don't "brake" anywhere near enough for it to be beneficial.

Normally not: Extracting energy from the airflow produces drag, which must be overcome by added thrust. Since every form of energy conversion produces losses, more thrust energy must be added than can be gained from the airflow.
Only when the engines fail and the generators stop running does it make sense to extract energy from the airflow. In airplanes there are two applications which are driven by "wind energy":

Older airplanes use gyros for the artificial horizon which are powered by ram air. This way the gyros work even after an engine failure.
Jets use Ram Air Turbines (RAT), propeller-driven generators which are moved into the airflow when all other means of generating electricity and hydraulic power have failed. Note that this is done in unpowered flight and increases the sink rate.

Deployed RAT (picture source)
Rocket-powered airplanes have no easy way to generate electricity, so the Me-163 B used a small windmill at the tip of the fuselage to drive a generator.

Me-163 B (picture source)

EDIT:
With your question's new focus on electric propulsion the answer will become different. Now you most likely will have propellers which are powered by electric motors. During landing, those could run inverted and charge a battery, which will most likely be empty by the time the aircraft lands. This can be done during final approach until the end of the runout after touchdown. It can be expected that any electric airplane will have a high L/D, so deploying speed brakes to allow for a steeper approach makes sense.
I would be surprised if an additional device would be economical, however. This recharging must be done by the regular propulsion system, or it would add dead weight during most of the flight.
You asked for formulas, but all I can provide here are some back-of-the-envelope calculations. First, it must be said that variable-pitch propellers will be lousy as windmills, because their camber and twist are wrong for windmilling mode. I would expect that their efficiency is around 30%, which means that only 30% of the energy extracted by drag will be converted into mechanical energy which drives the electric motor.
Next, running an electric motor as a generator will again require compromises. Good motors make poor generators, and re-wiring the motor for better generator performance will degrade its efficiency in normal use. You will quickly lose more than you gain from the short flight phase when running the motor inverted makes sense.
Now let's assume that you keep your propulsion system at top efficiency (say, 90%) and accept that it will only convert 10% of the drag energy into electric energy. Let's also assume that your prospective solar airplane has an L/D of 30 which needs to be reduced to 10 for a practical approach. You do this from 1000 ft downwards and use the windmilling propeller also during the runout. Approach speed is $v$, mass is $m$ and the initial energy of the airplane is $305\cdot m\cdot 9.81 + \frac{m}{2} \cdot v^2$. Two thirds of the potential energy go into propeller drag, and to be generous we assume that 100% of the kinetic energy will also go into propeller drag, even though the braking force of the propellers at low speed is really lousy and needs support from wheel brakes.
Now it is important how fast your airplane flies, because this will shift the ratio between potential and kinetic energy. To keep things simple, I will relate both to the energy required for the next flight. 10% of the full kinetic energy will accelerate the aircraft to less than one third of its flight speed - after this, the remaining 91% of the energy to reach $v$ must be added by charging the batteries between flights.
The electric energy taken from the potential energy will help you climb to 60ft or sustain level flight at $v$ for a distance of 1800ft. At an L/D of 30 the airplane will fly a distance of 30,000ft without thrust, and by braking you will extract the energy to cover 20,000ft, which at 10% conversion efficiency (and 90% propulsion efficiency!) will carry you over just 1800 ft.

Perhaps in the future lightweight RATs could be deployed when speedbrakes are normally used. The electricity could then be used to charge on-board batteries. Electricity could then be used to taxi with the engines of. An airbus A320 RAT provides about 40kW of electrical power at 130kts. Running 4 of them for 2 minutes, would add to about 5kWh. Probably 100kWh would be needed to taxi a couple kilometers, so it doesn't seem that feasible. Might be a better idea to just put bigger  generators on engines and let them generate more electricity when the engines are on idle.Might save fuel, might not

Yeah, it appears that wind turbines can act as better airbrakes compared to spoilers. In other words, it is called regenerative braking in electrical science. But this must have got some negatives and hence has not been in practical. Please consider this and let me know why it hasn't been yet implemented.

Among other things, the general goal pilots of airliners tend to shoot for is an idle power descent, where the engines are at idle from top of descent to some reasonable point on final approach (in reality, ATC considerations mean you won't always be so perfect). In other words, the aim is to avoid using speedbrakes unless necessary. With your plan, you'd have to use the engines more, consuming more fuel, so that you'd be able to use your turbines to charge a battery. That would be horribly inefficient.

Another way to think about this is from the perspective of energy management. An aircraft at cruising altitude has a certain amount of potential energy. It can set the engines to idle (because regulators and passengers frown on turning them off completely in flight) and glide down to the ground (or to glideslope intercept anyway), just as a car can eventually coast to a stop if nothing is in the way. Your turbine would take energy out of the system, but that energy has to come from somewhere. The only source of more energy is the engines and fuel already on board.

Cars may use regenerative breaking because roads aren't designed to allow everyone to coast to a stop without crashing all the time, but cars are not airplanes and the sky is not a road.

@ZachLipton: As a physicist, I understand your concern for the law of energy conservation. Producing electricity from an idle engine in descent indeed requires energy, but you'd steal it from the trust. Engines produce far too much trust in descent anyway. This is because you can't spool up an engine quickly for a go-around. So, you produce far too much power, and this can safely be captured. I wouldn't bother with a RAT, though, and steal the power directly from the engines. Air is not a good medium to transfer mechanical energy.

I would venture that the engines are capable of providing more than enough power during flight to charge whatever batteries might be needed for electric taxi once back on the ground. The drawback (at this point), is toting the weight of the batteries and motors needed for electric taxi vs the extra fuel burnt while on the ground.

@Peter Kamp F. Your edit really is sensible. But please explain it little more in detail with the help of calculations if any possible.

@B Uttej you can not substitute spoilers with a RAT, because a spoiler has a double function, increase drag and destroy lift. The latter one is paramount in descent situation, and is used to achieve high rolling rates as well!

@BUttej Yyou can not substitute spoilers with a RAT, because a spoiler has a double function, increase drag and destroy lift. The latter one is paramount in descent situation, and is used to achieve high rolling rates as well!  Moreover consider how quickly a spoiler can be retracted actuated, when you want to stop the "breaking" effect. Compare it with the current RAT kinematism (door opening etc)

RAT's may also be used for specific purposes that don't involve the actual flying of the plane.  For example, certain models of the KC-135 aerial refueling planes have outboard fueling modules on the wings, and the pumps and hose drums on these are powered by a RAT.  It does cause increased drag, but that's okay because they're not operating the whole time, only during fueling operations.

Most other answers focus on normal flight; your (updated) question specifically asks about regenerative braking. In theory, yes, it's possible, in practice, no, it's really not a practical idea.
Let's first focus on the descent from cruise altitude to final approach. Ideally, this descent is done with engines on their lowest thrust setting (flight idle), which means the engines are providing power to the various electric and hydraulic systems, and pressurization, and a little bit of thrust since you can't really avoid that on a jet. Theoretically, this could be done by a Ram Air Turbine (let's for now assume that our hypothetical airplane is equipped with a rather big version that comfortably powers all aforementioned systems, since a typical RAT can only provide limited emergency power). However, this will greatly increase the drag, and to maintain speed, the descent profile will be steeper. This in turn means that the airplane must spend a longer time at cruise altitude, which requires energy to maintain.
Another way to look at that is by a simple energy balance: there is a finite amount of potential energy, and it doesn't really matter whether you use it to maintain speed only, or whether you use it to maintain speed and for regenerative braking, since the total amount of energy you can extract from it is always the same.
Of course, there is one moment that airplanes must brake, and this is after touchdown. Let's for now assume that we are very environmentally aware, and do not use any other means of braking than a regenerative braking system. The airplane is still going at about, say, 130kts, 240km/h or 150mph, so surely that will give us a lot of energy? Let's make a back-of-the-envelope calculation for some figures I found for a Boeing 737-300.
Let's say we land weighing 60 000kgs (close to MLW) with flaps reduced to 15, which gives us a landing speed of 158kts=81,3m/s. The amount of kinetic energy is then $\frac{1}{2}mv^2\approx200 MJ$ (yes, that's mega-joules!). That's a lot of energy, right?! Well... not really. Kerosene (which is approximately the same as jet fuel) comes at an energy density of around 46MJ/kg. We're talking about the equivalent of a little over 4kg of kerosene, on an aircraft that carries about 16,000kg of the stuff. That means we're regenerating about 0.025% of the fuel capacity.
I will leave it as an exercise to the reader to think whether a regenerative braking system could be designed, such that the extra fuel used due to its weight and size is under 0.025%.
Edit Let's redo the calculation on an airplane that actually has most of the necessary equipment (batteries and electric motors that could perhaps double as alternators or dynamos) on board: the Solar Impulse 2. It has a whopping 4x41kWh battery capacity (590MJ). Assuming the takeoff and landing speed are the same (20kts=36km/h=10m/s) and with a loaded weight of 2300kg, the kinetic energy upon landing is 115kJ. This is 0.0195% of the battery capacity - about the same as our B733 example! And note that this is again assuming that the propellers recover 100% of the kinetic energy... This idea is never going to work. (For comparison, it's less than two seconds of energy generated by the solar panels at their respective peak rating)

It's also good to realize that we're talking about $2 or so in fuel, per landing. How much is a certified system going to cost?

That means we're regenerating about 0.025% of the fuel capacity. That right there is the #1 reason this hasn't and won't be implemented. It's simply not worth it in terms of dollars, economics, or environmental concerns (don't forget it takes natural resources to produce all those regenerative braking components & especially batteries).

@FreeMan Especially since kinetic energy is quadratic in speed. For a more realistic landing (40 000kg, 115kts@flaps 40) we would have a kinetic energy of only 70MJ. That's the equivalent of serving tea on a British Airways flight (i.e., 1 liter of tea per passenger)

@sanchises even with a BA flight full of Brits, I somehow doubt they'd get through an average of 1 litre of tea per passenger. I would do my bit to help though.

Sure, equipping a jet with such a system seems inefficient. But what about propellers? They seems much more appropriate for the job – they can be powered by electricity alone, and a dynamo is basically an electric motor working in reverse.

@Édouard Commercial aircraft will not be powered by electric motors until batteries get a lot better. Seriously, even if they were, I think the weight of the electronic components required for properly storing the recovered energy would impose a fuel penalty greater than aforementioned 0.025% (and have you ever seen a propeller aircraft land at 158kts? and how about a turbine with 100% efficiency? See the comments above; I think for a propeller craft, we're talking about 10-20MJ tops.) I'm sorry, but this idea is completely infeasible.

The OP doesn’t mention commercial aircrafts either. I’m not saying you are wrong (I think you’re right, though I quite frankly do not have any kind of competence in the matter), but that your answer assume specifics which goes against the premises of the question. Or, if you’d like, you say “you can’t run” when you could probably say “you can’t even crawl”. Which would make your already great answer even better.

@Édouard How about this?

No, because, from the point of view of a flying aircraft, wind does not have any energy.
Wind turbines stand on the ground and the air mass is moving at some velocity past them, so it has kinetic energy. But aircraft is moving relative to the wind, so the wind is the rest frame and has no energy. So when the aircraft is using the air flow and/or ram air pressure, it is using its energy, not the wind's¹.
Now of course a turbine mounted on aircraft will produce energy. But it will be at expense of the aircraft energy. When flying level under power, the power comes from the engines, so it is more efficient to extract it directly via generator mounted on the accessory drive. But even when the aircraft is descending on idle, it will be at expense of its potential energy and that was originally provided by the engines during climb. Shutting down the engine earlier and using all of it to compensate for drag during the glide is more efficient.
This applies to a solar powered motor glider just like any other plane. It is more efficient to stop the engines earlier and glide at around best glide angle than running the engines longer and then regenerating the energy, because neither conversion from electric energy to potential energy via engine and propeller, nor the conversion from potential energy to electric energy via turbine and generator is particularly efficient.
And it applies even when using thermals or other raising air to gain potential energy. It is again more efficient to avoid converting the energy and simply use the thermal to gain altitude and extend the glide while taking electric energy directly from the solar panels.
Also, weight will be important limiting factor for solar motor glider. This means you won't be able to put many batteries on board and that in turn means that regenerative braking won't be very useful as you won't have much capacity to store the energy. It also means you might want to avoid dedicated turbine to save weight; you'll still be able to regenerate some energy from windmilling propellers at somewhat lower efficiency. But as explained, it does not make all that much sense.

The only time aircraft use turbines is during emergency. When all engines fail, ram air turbine is used to power the essential electric and hydraulic systems. It shortens the gliding distance a bit, but it is worth it if no other power sources are available.
As far as wind goes, the only practical use of wind is choosing route so that there is as much tail wind during cruise as possible. For example the North Atlantic Tracks are periodically adjusted to allow the eastbound flights to utilize the Jet Stream. In a sense, this utilizes the wind energy, because the aircraft thus burns less fuel to get to its destination.

¹ Energy is a weird quantity. It is conserved in all inertial reference frames, but some of its forms will have different values in each. You can choose a reference frame where wind does have energy, but it will make less sense.

@mins, well, yes. It would still be at the expense of potential energy and therefore gliding distance though.

@mins, of course it does. But if you have a wind turbine and extract energy, it increases more slowly than if you didn't.

@mins, it is always more efficient to use the energy from thermal to gain altitude and thus save fuel than to extract energy from it via RAT.

Actually the idea is to build an electric aircraft whose power comes from solar and from wind while the plane is landing. instead of using spoilers as airbrakes, these wind turbines separately mounted could add drag and harness energy at the same time . In other words, regenerative braking. Please help with this

@BUttej, I would expect the solar panels to be more efficient, but I didn't do any calculations. That would involve quite a lot of advanced physics. The problem with regenerative braking is that due to weight constraints you won't be able to carry much batteries, so you won't be able to store much of the energy.

right now wind turbine efficiency is 38%, while solar has 20%. So how can solar panels be more efficient?

@BUttej - your comment about regenerative braking really should be in the question, to make it clear what you're talking about. I think at the moment nobody is answering the question you really meant to ask.

@Andy Thanks for your suggestion

@BUttej, right, it didn't make much sense. Actually, comparing efficiency does not make sense here. The thing here is that if you want electricity, the solar panels make it from external source, for which you have no other use, so it basically has no cost, while the turbine takes it from your potential energy budget, which can be more efficiently used for staying aloft. Even during the descent, it is more efficient to stop feeding your potential energy budget earlier and store the energy produced by the solar panels than to run engine on solar power and then do regenerative braking.

@BUttej, updated the answer.

North Atlantic Tracks are periodically adjusted to allow the eastbound flights to utilize the Jet Stream.   ...and westbound flights the reverse - to avoid the Jet Stream. http://aviation.stackexchange.com/q/25012/3573

@Jan Hudec, re: your point about thermals. I am imagining a method where a regenerative prop braking system could yield useful energy: Enter a thermal, ride to the top, exit, switch on the regenerative system while descending to the bottom, re-enter the thermal and repeat until the batteries are fully charged. Is there a flaw with this that I’m not seeing?

@Seb, it would work; it would be rather complicated though and the batteries are heavy, which limits how much energy you can store.

For the reasons other answerers have said, it isn't normally worthwhile, because it isn't really wind energy, it's the aircraft's energy.
One situation where it is worthwhile is to generate electricity when it is difficult to take energy from the engine directly. For example, the vintage aircraft I fly didn't originally have an electrical system. To power their radios and transponders (essential for practical flight nowadays), they've been retrofitted with a small turbine under the nose. It increases the drag of the aircraft slightly, but the alternative would be trying to fit an alternator to a vintage engine, or replacing the engine completely, both much bigger modifications.
I've also seen touring aircraft which use small wind generators when they're tied down on the airfield, presumably to keep the battery topped up. The generator in that case is a small wind turbine atop a vertical pole, similar to what you might see on a houseboat or a caravan. They're removed and stowed before flight, so they're not quite what you're thinking of, but they're still aircraft using wind power.

I think no one really adressed your question. I'll try to, as far as I understood. (I still wonder why you need a plane landed with non-empty batteries, but let's assume it's for faster refuel & takeoff?)
First of all, it is only possible to harvest wind energy (of air moving w.r.t. ground) if you have access to the ground.(You need to be a the interface between the two moving objects). A wind turbine must be grounded, a sail boat is in contact with the water, etc. For an aircraft this is not the case obviously, so as said in other answers it is impossible to gather free wind energy mid-flight.
However, it is possible to use windspeed to harvest some of the aircraft's energy. Let us recall there are three main energy tanks in an aircraft (i'll use A320 and TB20 (sorry french link for numbers) both in cruise in a regular mission, it would scale nicely for any e-aircraft):

Kinetic energy (momentum) - A320=2.2GJ / TB20=3.5MJ
Potential energy (its height in the earth gravity field) - A320=7.7GJ / TB20=66MJ
Fuel energy (required to cover least 1000 n.m. against the drag force, plus the above) - A320=180GJ. / TB20=1000MJ

As you can see, fuel energy in a classic airplane really dwarfs potential energy about 20-to-1, and potential surpasses kinetic about 10-to-1. It's the travelling part that costs the most, and unfortunately, airplanes are kinda designed expressedly for travel :D
Since what you used to travel (against the drag) can never be recovered, only the surplus kinetic and potential at the end of the mission may be. You propose to perform the descent and landing with the wind turbines. Let's suppose you have the wind turbine for the job. You may only re-gather kinetic energy during approach and landing, and potential energy during descent at constant-speed.
Even if you manage to recover the whole energy, you only get ~5% of what you used in your whole mission! (and i used 100% efficiency everywhere, full system should be close to 20% efficient including turbine, generators, power unit, batteries etc. so we're really speaking about 1% net energy return).

That said, there might be a use for a windspeed turbine in very specific missions, but you would prob. drop the solar panels. I'm thinking parachuting planes. Mission: go high very quickly, drop your buddies, descend, repeat.
For this the energy demand isn't very high since you're not travelling.
I'm thinking a mission requirement of 2x pot. energy to:

Get to speed
Climb to altitude
Beat the drag while climbing but not travelling
Then you'll have 1x pot. energy + 1x kinetic energy to recover, and the numbers change a bit: there is 50% of energy available to recover, so with 20% efficiency, about 10% of the whole input can be brought back down.

I'm saying drop the solar panels, because this will be very power-intensive, and
solar panels just have too low energy density
Note: energy density of a system is energy packed/mass on board. For batteries and fuel this is straight forward, for solar panels it's different: the longer the mission, the more energy produced, the higher the power density. This mission is so short it's not worth it.

TL;DR : Unless you have a very weird mission, there just isn't enough energy (~1%) to collect to be worth bringing a few wind turbines for a whole travelling mission.

Airplanes move pretty fast (in some cases very fast) and have lots of drag. So they use lots of energy just to cruise.
Regenerative braking allows you to capture, as an absolute upper limit, all the kinetic energy and gravitational potential energy that the plane has when it starts its descent (obviously there's then an efficiency consideration too, so it will actually be less than this). The work you've done during the flight just to overcome drag at cruising speed is gone no matter what. So first thing: any drag that your doohickey adds to the plane while it's not braking, is costing energy through the whole flight. Let's assume that it's somehow stowed away, just like landing gear can be, and figure that it's going to bulk the plane up a bit but it isn't going to totally ruin the aerodynamics.
Now, on a typical flight what proportion of the fuel used, is used to reach cruising altitude and speed? I don't actually know, and of course it depends on the length of the flight among other things, but I'm sure some actual pilots could chip in with rough figures.
The energy to get up is an upper limit (again, there's inefficiency in the engines) on how much energy you can extract from the process of getting down. So, that proportion of fuel used to get up puts an absolute upper limit on the proportion by which regenerative braking can possibly reduce the total energy requirements of your solar-powered plane. And that's before considering inefficiencies of both processes (the engines and the regenerative brakes). So to completely invent some figures, supposing take-off is 25% of fuel for some particular journey, and combined efficiency is 50%, then regenerative braking could perhaps reduce energy requirements by 12.5%. This seems worthwhile on the face of it, but (a) I've picked what I think are overly-large numbers, and (b) we haven't yet paid for the mechanism that does it.
Can the batteries (or whatever) that store the energy from the brakes, store more energy than it costs to add them to the plane, thereby hauling a larger weight up to altitude in the first place, and the energy lost in cruise to the extra drag imposed by incorporating the whole system? In the worst case where they can't, you've made a net loss by adding regenerative braking.
Regenerative braking works reasonably well for cars, especially in city driving, because they slow down frequently and so they're otherwise dispersing a lot of unwanted kinetic energy as heat. To a rough approximation, planes only slow down once per trip. And I believe it's harder to efficiently capture the energy when you're braking against air rather than against static friction on the road, and so the efficiency of the system on the plane will be much less than that for a car. So I don't think your plan is in good shape right now :-)

Very Short Answer

Is it sensible to harness wind energy on an airplane?

I assume as "wind energy" you mean, extracting energy from the "airflow". In theory yes, only in descent, if you can prove a design which brings benefits in terms of energy, complexity, economics. In practice, you have very slim margins with your solution, and only "one case" when harvesting energy makes sense.
Little Longer Answer
Let's analyze the idea:

... to build an electric aircraft whose power comes from solar and from wind while the plane is landing.

While the airplane is landing it is descending from "cruise" altitude to the airport. In theory it would need no "extra power", since you have a lot of potential energy, you could just use this potential energy to "glide" to the airport, similar on what sailplanes do. An important note the energy to get to "cruise" altitude has already been spent, for example, what Solar Impulse 2 do is to climb and recharge the batteries during the day, and glide and use the energy-from-the-batteries during night.
Don't underestimate the efficiency of the gliding maneuver over night, with altitude you gain a lot of potential energy! You would not gain the same energy by harvesting it to from the airflow because you are transforming this potential energy in electrical energy with a transformation chain that bring you some losses.
That said, the one case when makes sense to harvest energy in descent is the moment that you need to loose some altitude in a "short" period of time, (not glide to the ground, which would be the most efficient solution). In this case it is desirable and possible to "harvest" some energy, what you are proposing is:

instead of using spoilers as airbrakes, these wind turbines separately mounted could add drag and harness energy at the same time . In other words, regenerative braking.

This is correct, it is done in a similar way with Ram Air turbines in emergency situations (no power) on larger jets. On a variable-pitch propeller aircraft theoretically you could change the pitch of the propeller to make them work as windmills, so that you would not need to add extra complexity and weight. If you have an electric motor driving the propellers you could potentially using it as a generator.
 In case you add "wind turbines", or any extra system, you should make sure that:

The energy to fly the extra weight of the system during all the mission is less that the energy that the system helps you harvesting
The cost of the harvested energy helps you breaking even the cost of the system in a reasonable amount of time.

A last note: Spoilers are use not only to add drag but even to reduce lift, the main use case here is descent (emergency or not)!

My company has just secured a patent for an Energy Cell Regenerative System For Electrically Powered Aircraft (May 9, 2017).
A properly designed and efficient regen system can flip the prohibitive physics enough for a net gain and be a worthwhile addition to an electric aircraft. A key is to design a lower pressure area beneath a cowling venturi  and harness some of the prop blast in flight (there is some) it tornadoes around the fuselage but beneath the cowling is,combined with the speed of the aircraft, a source of kinetic energy.
The turbine is a centrifugal design and is tucked away under a blister which makes the system nearly drag free when not in use.  The PMA can produce 3kw and weighs 19 pounds. The whole system weighs 31 pounds and includes a mosfet controller and a buck boost inverter similar in design to those found on Toyota's Prius. There is a lightweight Maxwell super capacitor module as a "charger".  Power generated from the system charges a probably lithium battery pack that has been designed to be charged and discharged simultaneously  (see the patent), through an algorithm managed by the controller. The lighter we can produce the system and also speed up charging by reducing the battery's internal resistance the more efficient, and practical, it will be.
We haven't been permitted to flight test it, but calculations derived from a Cessna 152 mule indicate good potential for regen. It can be used as a rescue system, during descent, or when practical to adjust prop pitch.
Knee jerk reaction from laymen and engineers has been negative until they carefully study the invention, then consensus seems to be "That should definitely be developed further".  Patent is on the market.  Patent #US 9,643,729 B2

The normal way to harness the energy of the air in an aircraft is a glider.
The wikipedia article covers it pretty well: find an area where the air is rising, and use it to lift the aircraft up. There is no fancy energy conversion on the aircraft, it's all done directly by the lifting surfaces.
Solar-powered planes have been built, although the necessary compromises mean they're not yet popular. Some tech companies are looking at the possibility of high-endurance automated solar aircraft as radio relays for connecting remote areas to the Internet.

Downvote because the information provided in this answer—while interesting—does not address the question: how to harness the energy of the aircraft's forward motion through the air.

Upvoted for thinking outside the box. Gliders do indeed harvest energy from wind. Vertical wind, that is. Any electrical airplane could do worse than using this energy. Electric airplanes have low energy density, so they fly slowly and have a glider-like shape. Flying like a glider would make most sense for them.

Aircraft generate lift by increasing airspeed, while wind harvesters generate power by decreasing airspeed, so they'd be competing with each other.
Assuming that there was a way to avoid that problem, it'd also need to store this energy, and large capacity batteries are incompatible with an aircraft's requirements for both high specific energy and high specific power.

This will be built-in, and almost mandatory.
Your question has a built-in assumption: regenerating energy is useful.  The only way it would be useful is if the propulsion system (or part of it) was electric. Therefore by definition we must be referring to an aircraft with an electric propulsion.
Also presumably, we are not throwing 1880-era tech into the sky. That means we are using AC motors with flux vector drive, i.e. Large semiconductors synthesizing 3-phase sinewaves from DC.
If the drive frequency is the same as motor speed, that is inert - no current flows and no thrust is made.  If a higher frequency, this applies power.  If higher still, it applies more power.  If slower, it regenerates.   If slowerer, it regens more.
So if you have an AC electric drive, you already have regen.  Done!
It'll essentially be thrust reverse.  Where do you get to use that?   Glide slopes are designed very shallow so you'll usually need power all the way down.  Look at how they fly the C-17, they would need to redesign approaches to be like that, which would also make them exclusive to electrics and thrust-reverse-in-flight machines like the C-17.


text:
Can the need for an engine overhaul affect the ETOPS rating?
I read this news:
http://edition.cnn.com/2016/04/23/us/boeing-dreamliner-engine-fix/index.html
So seemingly the reliability of the 787 engines made by General Electric is not as high as expected and need to be overhauled to increase reliability. If one engine fails due to icing there should be also increased chances that the second one may fail as well.
Does it mean that - even temporarily - this may have an effect on the plane's ETOPS rating?

Every time an engine is shut down in flight, it is tracked.
Engine reliability does affect ETOPS certification. In addition to the aircraft being certified, each airline is also ETOPS certified. An aircraft or an airline with a poor record could have its ETOPS certification downgraded, or even potentially revoked.
Wikipedia ETOPS Approval

ETOPS approval is a two-step process. First, the airframe and engine combination must satisfy the basic ETOPS requirements during its type certification. This is called "ETOPS type approval". Such tests may include shutting down an engine and flying the remaining engine during the complete diversion time. Often such tests are performed in the middle of the ocean. It must be demonstrated that, during the diversion flight, the flight crew is not unduly burdened by extra workload due to the lost engine and that the probability of the remaining engine failing is extremely remote. For example, if an aircraft is rated for ETOPS-180, it means that it should be able to fly with full load and just one engine for 3 hours.
Second, an operator who conducts ETOPS flights must satisfy their own
  country's aviation regulators about their ability to conduct ETOPS
  flights. This is called "ETOPS operational certification" and involves
  compliance with additional special engineering and flight crew
  procedures in addition to the normal engineering and flight
  procedures. Pilots and engineering staff must be qualified and trained
  for ETOPS. An airline with extensive experience operating long
  distance flights may be awarded ETOPS operational approval
  immediately, others may need to demonstrate ability through a series
  of ETOPS proving flights.
Regulators closely watch the ETOPS performance of both type
  certificate holders and their affiliated airlines. Any technical
  incidents during an ETOPS flight must be recorded. From the data
  collected, the reliability of the particular airframe-engine
  combination is measured and statistics published. The figures must be
  within limits of type certifications. Of course, the figures required
  for ETOPS-180 will always be more stringent than ETOPS-120.
  Unsatisfactory figures would lead to a downgrade, or worse, suspension
  of ETOPS capabilities either for the type certificate holder or the
  airline.

Although perhaps worth noting that in this case there could potentially be a different ETOPS rating for GE and RR engined 777's, in which case we'd potentially see airlines re-engine their aircraft: whether it would happen depends on whether the cost of changing the routes outweighs the cost of switching

authors: Лачин Лачинов


text:
Can TCAS and GPWS work at the same time?
A manoeuvre to avoid a collision with terrain, might put you onto a collision course with traffic above you.
Can TCAS and (E)GPWS work at the same time? Are they coordinating?

Do you have a valid reason to believe that they might not?

(See chat -- https://chat.stackexchange.com/transcript/message/61400721#61400721 and subsequent (may wrap to "next day"))

Yes they can work together. It's not particularly sophisticated. Essentially there is a priority list for all alerts.
For TAWS/GPWS and TCAS, the TAWS/GPWS alerts will take precedence over a TCAS RA (the ground isn't going to move out of your way.) When the TAWS/GPWS issues an alert, a signal is sent to the TCAS to force it into TA-Only Mode.
When a TCAS is in TA Only mode, it can only issue TAs which are advisory and require no action. It will also communicate the fact it's in TA-Only mode to any other TCAS equipped aircraft that may present a conflict. If a conflict does arise, the TCAS (on the terrain avoiding aircraft) will generate a TA and the other aircraft will generate a non-cooperative RA. A non-cooperative RA is the same as what happens when a conflict arises with a non-TCAS equipped aircraft.

Is there any means via which the crew of an aircraft can indicate to the TCAS system that their ability to maneuver the aircraft is extremely limited (e.g. due to mechanical problems)?

Like, not reacting to the TCAS? It will eventually get it, and reverse the advisory on the other aircraft.

@supercat Yes. The crew can manually select TA-Only mode for the TCAS. That mode information is sent to any nearby TCAS-equipped aircraft that are potentially in conflict. Normal TCAS conflict resolution will result in coordinated RAs in both aircraft. TA-Only mode tells the other aircraft they have to resolve the conflict without a coordinated RA. Their TCAS assumes the TA-Only aircraft will continue on it's present track, updating its guidance as it tracks the aircraft.

@Haukinger The aircraft responding to the TAWS/GPWS alert will take no action based on the TA. Their job at that moment is to avoid hitting the ground. The other aircraft would get a TCAS RA that it would respond to to avoid the suddenly climbing aircraft. Once the aircraft has climbed sufficiently for the TAWS/GPWS alert to go away, their TCAS will revert normal TA/RA mode, at which time they could receive an RA if the conflict still exists. Since the TCAS gets raw Radio Altitude data, it won't issue a descend RA if it's below the minimum altitude.

For clarity, "TA-Only" is "Terrain Avoidance-only" and "RA" is "Resolution Advisory"?

@Gerry: Is switching of the TCAS system into that mode an early part of responding to unexpected control problems?  I know that the general rule is "Aviate; Navigate; Communicate", but I would think that communicating as quickly as possible a message "Look out everyone!  I can't control this thing!" should take priority over almost everything else, but should be easy enough as to not interfere with doing anything else.

@supercat I'm not sure about "control problems", but switching TCAS to "TA only" is on every engine failure checklist I've seen (since you cannot climb at your normal rate after a single engine failure).

@Bianfable: I was thinking of more general situations where the aircraft might become hard to control, or where a pilot might want to minimize stress on the air frame, e.g. because a pilot hears a loud bang followed by what seems to be excessive vibration.  Having pilots instinctively hit a button as their first action when something unexpected happens, even before they know what it is, would help ensure that they're not hit with a TCAS RA alert that other planes would expect them to follow.

@supercat There are some things you had better do right away as memory items (e.g. reduce AOA upon incipient stall), but it's hard for me to to imagine switching to "TA only" being one of those things, especially when you don't even know what kind of issue you're dealing with yet.

@WayneConrad: If switching would require navigating through multiple menus, having that be a priority might be misplaced.  If switching would simply require hitting a Big Red Button, then having a policy "if in doubt, HIT THE BUTTON" would seem pretty straightforward.  Switching TCAS to TA-only mode in a situation where it turns out to have been unnecessary may be discourteous to other aircraft, but shouldn't cause any harm beyond requiring a pilot to issue an official explanation as to why the button was hit.  Such a button could also help guard against loss of CVR/FDR recordings...

...in cases where a plane has a problem but ends up landing normally, and would generally seem advantageous.

@supercat I don't know all the emergency conditions that would justify switching to TA-Only mode, but as Bianfable indicated any time you can't climb as with an engine out it would be appropriate. An emergency descent may be another. As for switching modes, in most aircraft the mode selector is a rotary switch on the ATC/TCAS control panel in the center console (typically just below the throttles.) The typical switch options are TEST-STDBY-XPDR-TA Only-TA/RA so switching is one click of the knob.


text:
What are the data elements shown on the GE235 flight data recorder (FDR) plot?
The Aviation Herald already has an image of the Flight Data Recorder information from the crash of TransAsia GE235:

(click to enlarge)
I have determined that the green lines are for engine 1 and the blue lines are for engine 2, but beyond that, I'm a bit lost. Can someone translate the abbreviations and describe what information each graph is indicating for non-pilots.

Looks like they shut down the wrong engine after #2 failed.

Wow... that looks pretty damning.

I took the liberty of editing the title: this is only a specific FDR plot, a full FDR plot contains hundreds of signals not displayed here.

here's an annotated description of the FDR by an ATR captain: http://www.reddit.com/r/aviation/comments/2uyy3n/engine_flightdata_readout_for_the_crashed/codk873

From top to bottom:

main gear : whether there is weight-on-wheel not [scale on the left, in blue: "air/gnd"] (thanks to DeltaLima for pointing this out)
VHF1 : probably if there was communication of that radio channel (speculation on my side) [no scale]
Master warning : if the MW signal for engine flame out was active or not [scale on the right, in green : "fault/-"]
CLA : Conditioner Lever Angle [scale on the right, in green : "fuel SO/no FSO"]
Beta : usually is used to indicate aircraft sideslip, in this case is referring to the propeller blades, 90 degrees signal feathering. [scale on the left, in green : "90//10"]
Bleed valve : whether it was open or closed (as labeled on the right) [scale on the right, in green : "open/closed"]
PLA : power lever angle, aka throttle lever position [scale on the left, in green : "80//30"]
ITT : inter turbine temperature [scale on the right, in green : "700//100"]
NP : propeller RPMs as % of max RPMs [scale on the left, in green : "100//0"]
Fuel flow [scale on the right, in green: "2000//0"]
Objective TQ: objective torque (*) [scale on the right, in green : "100//80"]
TQ : torque [scale on the right, in green : "200//0"]
NL : low pressure shaft RPMs [scale on the left, in green : "100//0"]
NH : high pressure shaft RPMs [scale on the right, in green : "100//0"]
Oil pressure [scale on the right, in green : "100//0"]
Barometric corrected altitude [scale on the left, in cyan: "2000//0"]

*: from page 2 of that link:

Federico, I think Main Gear A/G does not reflect the state of retraction, but rather the state of the weight-on-wheels switch.

@DeltaLima you might be right, comparing to the adjusted altitude your explanation makes more sense than mine. Thanks!

Main Gear (A/G) is the signal of air/ground detecting solenoid that is in the main gear strut AFAIK, it detects when the aircraft is airborne and actuates several different systems that have different behavior while in flight than while on ground.
It is not Gear UP/DOWN condition, in most FDRs gear position are 3 different signals one for each gear.


text:
Why can the aircraft not turn on pitot heaters itself?
There have already been some posts about pitot heaters: when to turn them on/off, why not leave them on etc.  I don't want to go over that.
I am an engineer, so I'd like to ask:  if the flight management avionics detect an out-of-bounds disagreement from the pitot transducers, and the heaters are off, why not turn them on, in addition to whatever other warnings etc. they already perform?
An ECAM message about turning on the heaters could be made and, if the pilot explicitly wants the heaters off for some reason, s/he can instruct the computer or pull a breaker, so overriding the software.

Are you thinking of the recent incident in Russia, which happened almost immediately after takeoff? While speculation about causes of accidents is off topic on Aviation SE, the phase of flight at the time of the accident can make a difference as to whether automatic pitot tube heating would even have time to work, even if it would correctly detect the situation as it was developing.

This question is at least tangentially related as is this one

IIRC the reverse is true on B737: The default is on. If it is off by whatever reason a ECAM message is displayed. The crew can react to that message however they like.

There's much simpler and safer logic. Most modern Western transport aircraft automatically switch the pitot heaters on when in flight mode or at least one engine is running. Pilots can manually force pitot heat on, but there's no direct option to switch them off. Normal procedure is to leave it in auto.
Boeings since the 757/767 are automatic (exception: 737), and Airbus has had it automatic since at least the A300-600. ERJ is auto, not sure about the CRJ200, but the 700 is auto.
Pitot heat indication systems are already required by certification standards. 14 CFR 25.1326:

If a flight instrument pitot heating system is installed, an indication system must be provided to indicate to the flight crew when that pitot heating system is not operating. The indication system must comply with the following requirements:
(a) The indication provided must incorporate an amber light that is in clear view of a flight crewmember.
(b) The indication provided must be designed to alert the flight crew if either of the following conditions exist:

(1) The pitot heating system is switched “off”.
(2) The pitot heating system is switched “on” and any pitot tube heating element is inoperative.

EASA CS 25.1326 is basically identical.
The Antonov 148 is certified to EASA CS-25, so it must have had such a warning system.

Is this EASA reg why the DA40 with the G1000 always has an annunciation on when pitot heat is off or malfunctions?

@Pugz Similar. DA40 is a normal category plane, certified under JAR/CS-23 (Part 23 for those in the US), so the relevant regulation is CS 23.1326, which also reads nearly identical.

is there a similar reg for the U.S. for normal category aircraft?

@Pugz 14 CFR 23.1326. It's been harmonized (harmonised)!

Thanks! What year did this go into effect? No normal category aircraft with an FAA airworthiness cert/type certificate I fly have this.

@Pugz Citation says Feb. 9, 1996, as part of harmonization with JAR. I don't know if it existed in a different form prior. Note that it is only required if the pitot heat is required (due to IFR or flight in icing; Part 23.1323). Also, if the aircraft was certified on an older basis, it may be exempt.


text:
Who operates the hoist in a Coast Guard helicopter SAR team?
Could you tell me who operates the hoist in the Coast Guard helicopter SAR teams?
Is it the A&P technician(Flight mechanic) or the Rescue swimmer(winchman)?
I'm interested to procedures from around the world, in particular inthe US, UK and Canada, but not limited to them.

It has to be the flight mechanic, because the winch needs to be able to hoist up the survivor while the rescue swimmer is still in the water

Rescue swimmers ride the hoist (sometimes); their job is to go swimming to rescue people (funnily enough). Therefore, they can't operate the hoist.

Are they flying the Jayhawk (H-60) or a Dauphin?

who operates the hoist in the Coast Guard helicopter SAR teams?

This depends on country and other factors, perhaps aircraft type etc.
In general, the winchman descends on the end of the winch cable and does not operate the winch.

In the UK, helicopter search and rescue were, until recently, mostly provided by the RAF. HM Coastguard HMCG (now part of Maritime and CoastGuard Agency - MCA)  operate SAR helicopters too and will mostly take over the SAR role.

The standard SAR crew is made up of four members: two pilots, one of whom is the aircraft captain, a radar operator who acts as the winch operator at the rescue scene and a winchman, normally trained to paramedic standard, who will supply immediate first-aid and recovery services at the rescue site.

From RAF Sea King

Sometimes the role is taken by aircrew normally responsible for loading/unloading:

If you are part of aircrew, you would be responsible for loading and unloading aircraft payloads, ... You would also take on extra duties according to the aircraft type and its operational role. For example, ... you might train as a winch operator for search-and-rescue operations.

From RAF non-commissioned aircrew

Other References

Search and Rescue Training
Working with Search and Rescue Helicopters
Coastguard search and rescue helicopter programme

Winchmen and rescue swimmers leave the helicopter during a SAR operation, because it's their job to go down and actually rescue people. So, they can't operate the hoist. The job falls to the flight mechanic, who's the only one in the chopper who can see the rescue scene.

Modern helicopters like the AW139 can be controlled by the hoist operator itself during the winching like you can see at minute 27 https://www.youtube.com/watch?v=bsxlYEOmfM0

You'll need to provide some kind of reference to back this up. And say which service/country you are talking about.


text:
Is it technically feasible to make a solar hang glider?
Is there any hang glider with solar sheets on top that can charge the e-trike or the e-Lift's battery in flight ? Just like the Sunseeker...

I think you've got your terminology crossed here (but it's not your fault - your sources don't seem to clear on it either).
A hang glider is by definition a solar vehicle: They're non-motorized, and the pilot keeps them aloft by seeking thermal lift (created by the sun shining on the ground and heating it up) or ridge lift (created by wind striking the side of a mountain/hill (ridge) and being deflected up.
Both of these rely on the sun (either directly heating the local ground, or unevenly heating various parts of the Earth's surface, ultimately creating wind).
What you're referring to would be a solar-powered ultralight aircraft -- basically a hang glider with a motor strapped to it, or possibly an electric powered paraglider.

One could conceivably create a solar-powered ultralight using flexible solar cells on the wing  - in fact a little Googling reveals that it has apparently been done with an electric paramotor (a type of powered parachute) (A little more Googling reveals this wasn't as cool as I previously thought - they seem to have been using a support vehicle to recharge their battery packs (with solar cells).
As far as I'm aware no such vehicle is commercially available at this time (May 2014), but the materials and technology to construct such a vehicle are certainly available (the folks over at the Experimental Aircraft Association could probably provide guidance on building something like this), and it wouldn't surprise me if commercial or kit-build solar ultralights become available at some point in the future.

I wish they had posted a clear image of the solar paraglider...

@AyanMullick I did a little digging and it turns out that solar paraglider isn't as cool as I thought it was (they didn't mount panels on the wing or cage - they hard a support vehicle doing the charging).  The principle of solar cells on the wing still seems sound though (effectively making the vehicle a motor glider - charge the batteries while the motor isn't running, draw from them when it is).

@voretaq7: i'm trying to define the relation between a paraglider and solar energy. Is the definition of solar vehicle actually correct? Wikipedia defines a solar vehicle  as one that can use solar energy directly. A hang glider or a paraglider can't directly convert the sun energy. It happens on the earth's surface.

@mxlian I am deliberately using the term imprecisely in my answer, but if you want to get really pedantic about the definition I would argue that the Wikipedia article is wrong in its premise: The only true "solar vehicles" are ones using solar sails. Anything else is doing some kind of conversion (and if conversion is OK at that point we're just artificially constraining the definition so it doesn't include an engine powered by Jet-A refined from oil made from dinosaurs that ate plants that grew because of sunlight millions of years ago :) )

@voretaq7, I think the definition is that conversion is allowed as long as it is happening on board, which sounds reasonably self-consistent to me.

By your definition a hang glider would simply fall to earth at 32 feet per second squared as soon as the sun set.

I don't believe it can be done. The area of a tandem wing isn't large enough. With solar power at 1kW per square meter that will be 15 horse power but the conversion efficiency is not 100%. It is more like 20% so you are looking at 3 horse power. A typical trike needs a minimum of a 25 horse power motor. For example the electric Icaro uses a 10kW motor and it's not even Tandem. The wing area would only be enough to provide 20% of the needed power.

Solar power on Earth peaks at somewhere between 400W and 1.3kW per square metre, in theory, at midday.
In practice, we can typically expect more like 40-70% of this to be available due to inefficiencies in the solar panels and the fact the wing wouldn't be pointing directly at the sun. This is discounting clouds, and only at midday. At any other time, the amount available is lower.
Taking the best case scenario (1.3kW at midday at the equator, 70% available) then we still only have ~910 watts, or a little over 1.2 horsepower, per m^2
Let's take a typical hang glider: a Wills Wing Alpha 210 has 19 square metres of wing area. That would give 17 kW, or 22.5 BHP. In theory, that's more than enough to get a microlight into the air, if you can keep the weight of the battery, motor, and solar panels down.
In practice, though, are you really going to be flying at the equator, with your wings perfectly level, at midday?
And more importantly, are you really going to get a 17kW motor, 20 m^2 of solar panels, wires, and a battery (pretty much required) into the weight of a hang glider that weighs, itself, less than a typical person?
It's theoretically possible to make some kind of small ultralight type aircraft solar powered, but probably not until we find a way to make lighter motors and lighter solar panels that can take advantage of the wing area without causing weight/stability issues

By solar hang glider I mean ;

the hang glider would take off on its fully charged battery
It'd have solar panels on the wing that would charge the battery while one is soaring with the engine off and also in normal cruise flight.

How substantially would that improve the endurance of that machine..

If you're just looking to use the battery to extend the range then that's more feasible - but the weight of batteries, solar panels and motors, even if the solar panel area required is reduced, would still be significant. Hard to say exactly, but I'd still put it under the 'not quite feasible yet' category

Even with these light weight flexible panels....?

Even if we believe their claims on the capabilities of those panels, which seem... Astounding... You'd still need well over 100 of their largest panel to power an ultralight

But with just the wing area with solar panels; wouldn't that charge up the battery considerably while the pilot is soaring with the motor turned off for a couple of hours...?

If you only need a few minutes prop power to gain altitude and have flexible lightweight panels on top charging the batteries for several hours then you could theoretically travel anywhere without an aerotow or hill thermal to get you up each time. The others replying here seem to be assuming continuous power capable of supporting the engine at full revs is necessary when actually you only require a few minutes to reach enough altitude. Then you can catch thermals and only fall back on the motor if you lose too much height.

These small motors taken from model aircraft give 10 minutes of thrust but can be used intermittently for hours. Combined weight of 4.4kg I think including the batteries. That is without adding a 1.8kg flexible 100 watt panel.

Welcome to [aviation.se]! Please expand on your answer with technical details to help it move from being a comment (and downvoted/deleted) to an actual self-contained answer.


text:
What is the maximum height of USAF fighter pilot?
In response to this comment:

Three days later I happened to meet a commercial pilot at school who has flown
  F4s during Vietnam. He gave me the worst news of my life. My 6'3" ass was too
  tall to ever fly a fighter.

Is this actually true? A cursory search seems to contradict what he was
told:

To become a Jet fighter Navigator, you have to have [...] a vertical standing
  height of between 64 inches (5 foot 4 inches) and 77 inches (6 foot 5 inches)
  tall.

It would appear his height is within range. Was he told wrong or am I missing
something?

Based on the answers below and the fact that the commentor was told by a Vietnam era pilot, I'd say they rules have likely changed since the mid- to late-60s.

Ron's answer is the correct answer, but just to ease your fears, I literally fly with Navy guys that are at least 6'3.

To become an Air Force pilot, you have to be a commissioned officer, there are few ways you can do that. First you can go through the Air Force Academy, which is probably the most common. Another way is to be in an ROTC program through your college of choice. The last way I know of is to graduate college and join the Air Force through the officer candidate school (OCS).
Regardless of the method you choose, there are certain requirements you must meet:

Be a U.S. Citizen
Have any 4 year college degree or be within 365 days of attaining it
Minimum 2.5 GPA (although I'd be surprised if they took you that low)
Under the age of 28 by the board convening date
Have a standing height of 64-77 inches and a sitting height of 34-40 inches
Have no history of hay fever, asthma, or allergies after the age of 12
Meet USAF weight and physical conditioning requirements
Normal color vision
Meet refraction, accommodation, and astigmatism requirements
Distance vision cannot exceed 20/20 uncorrected or must be corrected to 20/20 or better
Near vision cannot exceed 20/40 uncorrected or must be corrected to 20/20 or better

If you've met those requirements you can then go through the Air Force Officer Qualifying Test (AFOQT) and through Military Entrance Processing Station (MEPS) for your intellectual and physical evaluation. If you pass those tests, you may be selected for the Officer Candidate School (if not joining through ROTC or Academy).
After that you go into undergraduate pilot training (UPT) where you spend a year learning to fly through academic and hands-on training. Depending on how you perform at UPT you will receive a seat assignment. Higher scores are selected for fighter training while lower scores move towards transport aircraft. You can voice your opinion on what you want to fly, but ultimately the decision is driven by the needs of the Air Force at the time you graduate UPT.
After UPT, you will move to Advanced Flight Training (AFT) which is between 6 months and a year long depending on the aircraft you've been assigned. After AFT you'll be assigned a squadron and location.
If you don't meet height requirements
There is a waiver process, although getting a waiver for height requirements is extremely rare (if not completely unheard of). The issue is that the ejection systems are designed for a specific height. This process starts by appealing a medical review board which you are allowed to write a letter to argue your position, however even with a waiver you would probably not be assigned a fighter aircraft. You would probably be assigned an aircraft that does not have an ejection system like transport or mid-air refueling aircraft.  As RhinoDriver mentions in the comments, the T6 training aircraft is also equipped with an ejection seat and if you can't fly the trainer, you can't progress on.
The only other alternative to be a US Air Force pilot outside of height requirements is to be an unmanned aircraft pilot.
Source 1: U.S. Air Force ROTC Website
Source 2: Department of Defense Article

All USAF primary trainer aircraft are equipped with ejection seats. If you do not meet the physical requirements to fly in the T6 ejection seat then you'd never fly anything, including a transport.

Standing 64"-77"; sitting 34"-40"
More information: http://google.com/search?q=usaf+fighter+pilot+height+restrictions

Check with your recruiter (they are all skilled salesmen - as they get commission) - get it signed in writing that you are not too tall.  The cockpits of some jets are just too tight for some - at 6'1-1/2 - I was too tall for some.  Helicopters - no problem - and those old vietnam era ones are fun.  I don't mean to rag on recruiters - they can help you get into one of the service academies with some pull - they are usually upper level ranking enlisted (like master, gunny - etc.)  The two most enjoyable to ever have control over is that Harrier jet, and the Huey helicopter.


text:
What equipment is used by Boeing to convert the F-16 into an unmanned aircraft (QF-16)?
How exactly does this equipment convert the F-16 into an unmanned aircraft?
Is it possible to use this equipment on any other fighter (like the F/A-18 Hornet for example) in order to convert it into an unmanned aircraft?

Screw driver counts?

THE USAF has had a program for converting manned aircraft into unmanned ones for decades. The aircraft used so far include,

QF- 104 (from F-104 Starfighter)
QF- 102 (from F-102 Delta Dagger)
QF- 100 (from F-100 Super Sabre)
QF- 106 (from F-106 Delta Dart)
QF- 4 (from F-4 Phantom II)

Note that both aircraft are piloted. According to fencecheck.com,

(QF-4s are)..almost always flown with a pilot aboard, unless a weapons launch will occur. Usually he does not touch the controls but stands ready to take over if ground control is lost or the aircraft departs. The pilots fly the aircraft themselves on chase missions and to maintain proficiency.

The QF-16 replaces the QF-4. The conversion of (retired) F-16 to QF-16 involves the following steps:

Removal of non essential items like the Vulcan cannon and the radar (in previous versions like QF-4, the radar was simply deactivated)
Modification of the FCS (Flight Control System) so that both manned and unmanned flights are possible (previous drones were remotely flown)
Installation of Flight termination system to destroy drone in case of flight path deviation, which results in this:

Source: f-16.net

Installation of onboard telemetry systems to control the aircraft from the ground.
Installation of a scoring system to measure the missile success rate. This is required as most of the missiles are launched with dummy warheads or the flight path is programmed to evade the missiles, so that the costly aircraft could be reused. Only in a few cases are lethal missiles used.
The self protection suite (flares, chaffs etc.) are usually left on the aircraft to provide a realistic engagement scenario.
New paint job :), which looks like this:

Source: www.wired.com
It is possible to convert (any) other aircraft into drones. For fly by wire aircraft like F/A-18, only the software portion will change, but it is a minor thing.

@ aeroalias  So, will the unmanned F 16 be able to fire any missiles or drop bombs?

@CSinha No. I don't think that anyone would want their 'target' shooting back at them :)

Just FYI but this idea predates even the jet-age era aircraft you mentioned. I'm not sure if Operation Aphrodite, which turned older B-17's into one-way flying bombs, was the first instance of manned/unmanned conversion, as this tactic has also been used for ground-to-air target practice vehicles. With Aphrodite, the two man crew would take off in the aircraft, then parachute to safety, and then the plane was RC controlled from a "mothership". The concept was ultimately a failure, although certainly ambitious and ahead of its time.

@elrobis I've read about that operation. I think they even modified some aircraft. Also, it was for bombing and not for target practice. As you said, it was way ahead of its time and most of the aircraft didn't even reach the targets.

@elrobis those aircraft were not intended to return to base, they were effectively primitive cruise missiles. Not just the Americans tried it, the Germans also did using the Mistel program. While not in itself a success, it did provide valuable experience for developing the first purpose built guided weapons.

@elrobis, those were indeed “cruise missles” (and didn't seem to score any hits at all); however, radio-controller aircraft were indeed used for target practice even before. The term “drone” is even assumed to be a pun on the name of the first plane used for this, the De Havilland Queen Bee variant of the De Havilland Tiger Moth, a common training aircraft of the between war era.

I doubt it would be possible to give you a detailed and specific answer to this question (with a components list and such) - the US government and military contractors generally don't divulge complete plans and schematics for aircraft/modifications in active service.
In basic theory however, I can tell you that the F-16 is a fly-by-wire aircraft. Much like an Airbus aircraft, all flight controls are actually manipulated by the on-board computers, and the pilot is just telling the computer what they would like the plane to do.
Because it's a fly-by-wire aircraft all that's really needed to turn it into an unmanned aircraft is to change the source of the "pilot input" from a control stick and throttle handle to a radio receiver that's getting commands from the ground, effectively turning it into the world's most expensive RC aircraft. Presumably there are also failsafes in the event the control uplink is lost, and at least some basic security protocols to prevent someone else from sending commands to the system.
In the case of the QF-16 (which is designed to serve as a target drone for aerial combat training) not much more needs to go into the design - it doesn't need extensive fire control capability for example. It also gets some other features added which are designed to aid in scoring training exercises (determining the accuracy of the aircraft attacking the target drone).
If it were to go into a combat role it would require those features as well, and probably substantially more work in securing the control uplink so an enemy can't disrupt (or worse, hijack) control of the aircraft.

As to what would have to go into another aircraft to make it unmanned, that depends on the aircraft. Obviously the control uplink has to be installed so you can communicate with it from the ground. Control system modifications may be required as well, particularly if the aircraft is not "fly-by-wire" computer-controlled (the QF-4, which was the QF-16's immediate predecessor based on the F-4 phantom, likely required servos to operate the throttle and other flight controls).

Check out this article from Flightglobal on the Loyal Wingman program:
https://www.flightglobal.com/news/articles/pentagon-touts-loyal-wingman-for-combat-jets-423682/
It looks like Boeing and Lockheed Martin are in a position to provide a drop-in solution whereby you swap out a couple of LRUs (Line-Replaceable Units) and bang! You turned your aging manned combat platforms into an unmanned expanded magazine for your manned platforms. The QF-16 costs just over $1,000,000 for the conversion.
Given the size of legacy F-16 and F/A-18 fleets this seems like a practical solution for countries with constrained military budgets.

This does not answer the question. The question asks specifically about the QF-16, which is an unmanned target drone. The link is related, but appears to be about a modification to convert the F-16 into an unmanned weapons platform, a platform distinct from the QF-16.

This does not provide an answer to the question. Once you have sufficient reputation you will be able to comment on any post; instead, provide answers that don't require clarification from the asker. - From Review

Hey thanks for the tips! I'm new here and I definitely don't want to de-clarify anything. I will pay closer attention to the scope of the questions being asked.

No problem! This site does take some time to get used to. It is a bit different than many other forums or Q&A sites in that we really look for precision in asking and answering what is asked. Get to know the guidelines and enjoy what the site has to offer!


text:
Who pays for the damages in this Asiana-Aeroflot collision?
I just came across this article where an an Asiana 747 'collided' with an Aeroflot IL62 at the adjacent gate. As per the reports, IL62 had to be scrapped. From, what I read, it was like the Asiana pilot had a brain freeze.
I was wondering, in this case who payed for the damages and what happened to the pilots?
Here is the accident description from aviation-safety.net and here's a photo of the 747's wingtip embedded in the vertical stabilizer of the IL-62:

mind the main reason the Il-62M would be scrapped is likely that the aircraft was already old and scheduled to be taken out of service, they just did that early rather than repairing it for a few months.

Probable cause: excessive speed, according to NTSB.

The airline's insurance will have picked this up. Who else?

@SentryRaven I was wondering, which airlines insurance... because it was the fault of Asiana pilot.

Same as with cars: the offending party's insurance. So the Asiana insurance should pay.

Reading the ASN article and looking at the pictures, it says something quite positive about the strength of the 747's wingtips! Not pictured and barely mentioned was that the #1 engine of the 747 hit the wing of the IL62 prior to the wing-tail interaction. You can see the wing tip of the IL62 looking pretty ragged in the posted image, if you start looking closely.

Short answer:
Someone's insurance company is writing a check.
Longer answer:
What usually happens in the insurance world is both insurance companies write checks to repair their respective clients' aircraft.  The insurance companies may then fight it out between themselves to determine who is responsible for what, if it makes financial sense to do so.
In cases where injuries or deaths result from an incident it almost always makes sense for the insurance companies to fight it out(those payouts can run in the millions of dollars).
In cases like this where there's only equipment damage the cost of the repair will determine how aggressively Aeroflot's insurance carrier pursues getting their money back from Asiana's carrier.  If we're talking about extensive and expensive structural repairs into the millions of dollars Aeroflot's insurance carrier will be motivated to get someone else to pay the bill. Conversely if the insurance company can make their money back through Aeroflot's premiums and pursuing Asiana for negligence or something along those lines to try to get them to pay for the repair would ultimately cost more (money and time pursuing the case) than just absorbing the loss the insurance carrier will just absorb the loss.

authors: مشاري الرويلي


text:
Why aren't passengers sorted before embarking?
Boarding a plane takes a bit of time.
Especially if passengers enter the plane in a random order as those stopping to store their baggage might block the lane for others.
Why  airlines don't sort passengers, for example during the dead time in between checking boarding pass and actual boarding?

Possible duplicate of Would random sitting arrangement be the best way of boarding for passengers?

Many airlines do, to a degree.
That's why airliners are often boarded in sections (for example row 1-10, then 11-15, etc.).
Problem is many of the passengers don't listen to instructions, and rush the boarding agent the moment the lights go on indicating boarding has started, in part because they're afraid there won't be space for their 4 bags in the cabin, each of them oversized, if they're not the first on board.
And THAT's the real problem, many airlines not being rigorous about enforcing cabin luggage allowances and pushing people with excess to the back of the line and having them gate check (at additional fees) those bags consistently.


text:
Is there a minimum distance that a pilot is required to fly?
Say there are two airports in a city, a large one with a control tower, and a small uncontrolled airport. Say they're only a couple miles apart, such that the small airport is well within the larger airport's controlled airspace. Is anything stopping a pilot from flying directly from one to the other?

I often go flying without leaving the airport, sometimes I go up and just spend an hour in the pattern never going more than a mile from the runway.

There is no rule that says that a flight needs to be at least x miles long. Most training flights don't even leave the vicinity of the airport, and they still count.
The only thing stopping your hypothetical scenario would be that the pilot taking off from the uncontrolled airport needs to obtain an airspace  clearance before taking off. If the bigger airport is busy this could be denied, in which case the pilot would be stuck!

"... such that the small airport is well within the larger airport's controlled airspace."

@Dave Thanks, here in Australia it is called an airways clearance, but I have changed it as it looks like the OP is asking about the US.

@Nij well within can and often mean on the edge to the point where it depends what direction you go as to if you are in the airspace. Take for example KXLL which sits under the KABE shelf and depending on what altitude you climb to on departure or your direction of departure you may or may not be in the airspace.

@Ben no worries I figured it was something like that just wanted to make sure it was not a typo ill remove my other comment.


text:
Military paintjobs matte for airshow also?
Combat aircraft are painted in a matte paint to prevent them from reflecting moon/sunlight too much.
But what about liveries used for airshows? Those colorful paintjobs, are they also always matte, or are they sometimes painted in more shiny paint.

Not a direct answer, since she is no longer an active combat aircraft, but one of the past paint schemes you'll ever see is "Miss Demeanour". Certainly not matte!

Oddly enough, in WW2 the US Navy painted its night fighters in gloss black.  It was found during testing that it when a searchlight lit up a matte finish, it glowed a uniform grey, whereas a gloss finish only reflected pinpoint highlights and was actually harder to pick out.

Matte paint creates drag and reduces the top speed. The sheen of the paint is often adjusted to suit operational conditions. For example during WWII an aircraft being prepared for a special high-speed mission, such as a dangerous reconnaissance flight, might have its matte camouflage waxed over to give it that edge of speed. Many combat jets have routinely been finished in a semi-gloss, as a compromise between the two. Special schemes for airshows do tend to be high gloss.


text:
F-16: MAIN PWR ticking sound, what is it?
What is that sound that starts when MAIN PWR switch is turned on in the F-16? Its sorta like a fast ticking sound.
Why is it there, what makes it, when does it stop? And is it also there when only putting switch to BATT?
You can hear it here:

Sounds like the vibrator in an altimeter, but I'm not an F-16 pilot, so I'll leave this guess as a comment rather than an answer.


text:
How do you know ELT's batteries used more than 50% of their capacity?
Batteries in ELT need to be replaced or recharged when the transmitter has been used for more than 1 cumulative hour or when 50% of their useful life.
How could I know it's already 50% or more than 1 cumulative hour?

There are two ways:

First, when you test the ELT you should make a note in the aircraft logbook of when it was tested and the results of that test. You can also note how long the transmitter was on during the test. You can then look at all the logbook entries to verify that the cumulative time is less than one hour.
Some ELT's have a self-test feature that presents error codes via an LED or audible beep when they are powered on. For example, here is the self-test page from the Artex 345:

The ELT will beep/flash 7 times when the battery operating time is >1 hour.
As far as the useful life of the battery goes, that is determined during the manufacturer testing for the ELT and will be marked on the ELT as an expiration date. Here is an example:

Source: Aerotechservices.com


text:
How do radio communications work en-route for intercontinental flights over the oceans?
What is the altitude one must reach in order to communicate with the ground?
Is it possible to talk to ground via radio flying over the oceans?
How to know the exact position of an aircraft over the oceans?

Pilots make position reports over the HF radio. HF follows the curvature of the earth unlike VHF which is line-of-sight, so altitude should not be an issue.

As Ron mentions in the comments aircraft flying transatlantic routes (or really any long route for that matter) may use HF radios operating in the 3-30MHz range to communicate over long distances. HF radio waves have the ability to "skip" off the ionosphere if directed properly and thus propagate over the horizon and beyond line of site.
You can find some various frequency info here.
Honeywell also offers at least one satellite based solution.
You can also find a lot of good info here under the "communication" tab.
As for knowing the exact position of the an aircraft, this answers covers most navigation methods out there.
This thread quotes FL300 (30,000 Ft.) as the minimum altitude for continuous coverage across the Atlantic as does ymb1's answer here (route dependent).

That is very interesting and thank you for your promptness. Those extremely high frequencies waves must still encounter some limits though. Or are we saying that wherever an aircraft might be, as long as it keep 30,000 ft, pilots can keep contact with the ground during the whole cruise?

@cornutifelici HF (High Frequency) is 3-30 MHz, while VHF (Very High Frequency) is 30-300 MHz. HF is lower frequency (longer wavelength) than VHF.

Link-fix above for "communications", https://www.aopa.org/travel/international-travel/transatlantic then select Communications tab.


text:
Why are brake temperature limitations lower when the brake fans are running?
In the A320 FCOM it is stated that if the brake fans are running prior to takeoff then the brake temperature must be below 150 degrees Celsius. But if the brake fans are off then our limitation is 300 degrees Celsius. Any reason behind such different limits?

The fans cool not only the brakes, but also the temperature sensors, at different rates. The temperature difference is a measure to correct this anomaly. From the document  Proper Operation of Carbon Brakes by Airbus Flight Operations Support:

... the  temperature  indicated in the cockpit when the brake fans are running is significantly lower than the temperature indicated when the brake fans are OFF. This is because the brake fans are ventilating the brake temperature sensor that is not located inside the material itself. Therefore,
  as soon as the brake fans are running, the indicated brake temperature  decreases almost instantaneously.
... For all brake types, when  the  fans  are  running, the difference ranges typically  from  approximately  50°C  at  100°C actual brake  temperature up to about 150°C  at  300°C  actual  brake temperature.
... When fans are selected, the warning will disappear instantaneously,  but the takeoff must be delayed until the indicated temperature  consistently decreases below 150°C (for a warning set at 300°C)

@aeroaliasI still didn't fully understand what is meant by this.

@Jai sounds like the fan blowing across the sensors creates a falsely low indication, i.e. The brakes are still hot but the sensors don't read it correctly because of the cool air from the fans. Sounds like an odd arrangement to me. There must be some reason for it

The "odd arrangement" probably stems from the fact that they don't embed the temperature sensors within the rotor material itself since it would (most likely) require a wireless sensor to actually read the temp. Therefore the sensors are somewhere on the surface where they are affected by the moving air much more quickly than the core of the rotor is. Since this is known, they give a "fan-on" and "fan-off" target temp to account for the extra cooling of the sensor.